1. Field of the Invention
The present invention relates to an image processor and an image processing method for printing monochromatic images on a print medium using a black ink and a plurality of color inks containing pigments.
2. Description of the Related Art
An inkjet printing apparatus is capable of outputting high-resolution, high-quality images at high speed and has many advantages including low running cost and low noise during output. In recent years there are growing demands for photograph output of as high a quality as that of silver salt pictures and there are a growing number of circumstances where people print not only color pictures but monochrome pictures as well. In such an inkjet printing market, a pigment ink has come to be used more often than before, considering the color stability and fastness of output images.
However, printed materials using a pigment have conventionally been known to have a problem of so-called bronzing, in which a specularly reflected light makes the print look different from its original color. More precisely, in the bronzing phenomenon a reflected light, a hue opposite to a hue of ink color, that is complementary color, is observed in printed parts of a print medium. For example, in areas printed with 100% cyan a red reflected light is seen, giving viewers an incongruous impression.
To cope with such a bronzing phenomenon, Japanese Patent Laid-Open No. 2004-181688 has disclosed a method of overcoating an entire printed surface with a yellow ink that has a relatively low level of bronzing. In this method, the amount of yellow ink applied to an image area of the printed surface having a hue angle close to yellow (180°<H°<360°) is set smaller than that applied to an image area having a hue angle farther away from yellow (0°<H°<180°) in order to prevent the image from looking yellowish more than necessary.
However, the vigorous studies conducted by the inventors of this invention has found that in printing a monochromatic image using a pigment ink, the method disclosed by Japanese Patent Laid-Open No. 2004-181688, though it can suppress the bronzing, has resulted in a yellowish hue emerging over the entire image to an extent that makes the viewer feel incongruent.
It has been known that monochromatic images printed with only an achromatic ink, whether it be a pigment ink or not, become slightly tinged with color depending on the kind of a print medium. To deal with such a hue deviation, a technique has been proposed which, while using mainly an achromatic ink, mixes a chromatic ink in it for printing in order to get the hue deviation back toward the direction of achromatic color (for example, Japanese Patent Laid-Open No. 2005-238835). So, when a monochromatic image is printed using a pigment ink, it is possible to reverse the hue deviation toward the achromatic direction by overcoating the entire image with a yellow ink and at the same time printing another chromatic color ink.
However, to reverse the hue deviation toward the achromatic direction by applying an additional chromatic ink to an entire image already made yellowish by a relatively large amount of yellow ink requires applying an even greater amount of chromatic ink. In such a case, even a slight change in an ink ejection volume from an inkjet print head can greatly affect the hue balance, causing a hue that is outside the hue range permitted for the monochrome to be visibly observed by the viewer. Such an image impairment is hereinafter referred to as a “color rolling”.
That is, in a construction for printing a monochromatic image, it has been difficult to solve the two problems—the aforementioned bronzing phenomenon and the color rolling—at the same time.